Positron emission tomography (PET) gives an opportunity to quantitate
the expression of specific molecular targets in vivo and longitudinally
in brain and thus enhances our possibilities to understand and follow up
multiple sclerosis (MS)-related pathology. For successful PET imaging,
one needs a relevant target molecule within the brain, to which a
blood-brain barrier-penetrating specific radioligand will bind. 18-kDa
translocator protein (TSPO)-binding radioligands have been used to
detect activated microglial cells at different stages of MS, and
remyelination has been measured using amyloid PET. Several PET ligands
for the detection of other inflammatory targets, besides TSPO, have been
developed but not yet been used for imaging MS patients. Finally,
synaptic density evaluation has been successfully tested in human
subjects and gives opportunities for the evaluation of the development
of cortical and deep gray matter pathology in MS. This review will
discuss PET imaging modalities relevant for MS today.